Selective circuit for superheterodyne radioreceivers



Feb. 7, 1933. H. 1'. BUDENBOM SELECTIVE CIRCUIT FOR SUPERHETERODYNERADIORECEIVERS Filed June 29. 1929 L,

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#1 F J 1555? a (9 w J x 6 J MT 5 INVENTOH I H. IBUDENBOM A TTORNEVPatented Feb; 7, 1933 Hurrah STATES PATENT OFFICE HORACE T. BUDENBOM, OFSHORT HILLS, NEW JERSEY, ASSIGNOR TO BEIJL TELEPHONE I LABORATORIES,INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK SELECTIVECIRCUIT FOR SUPERHETERODYNE RADIORECEIVERS Application filed .Tune 29,

This invention relates to radio receiving circuits and more particularlyto receiving circuits in which a heterodyne wave is used in thedetection of the signals. It has for its principal objects theelimination of undesired signals due to the interaction of theheterodyne wave with other carrier waves than that to which the receiveris tuned, and also the maintenance of a uniform degree of selectivityover the tuning range of the receiver, including for all tuningadjustments, a selected frequency interval of sufiicient width to affordimproved fidelity in the re production of detected signals'as comparedwith results obtainable with tuning arrangements formerly employed.

It is well known that radio receivers of the heterodyne type may respondto signals from two different stations at the same time if the carrierWaves of these stations are spaced at equal frequency intervals aboveand below the frequency of the local heterodyne wave. Since receivers ofthis type are customarily arranged to operate with a heat wave of fixedfrequency, the heterodyne interference arises from stations havingcarrier frequencies differing by a fixed amount, equal to twice the beatfrequency, from that of the station it is desired to receive. This kind,of interference is particularly troublesome in wave length rangesoccupied by a large number of transmitting stations, as, for example, inthe broadcast range, in which the transmitting .wave lengths are spacedat intervals of about 10,000 cycles per second. Any attempt; toeliminate the interfering signal by changing the frequency of theheterodyne wave is likely to result only in the setting u a of a newinterference by another station. y sharply tuning the receiving circuitto the desired station the interfering signal may be d1m1ntive circuitsit is impracticable to obtain the high degree of discrimination neededfor the complete suppression of interference, wlthout impairing thefidelity of reproduction of detected signals.

Bythis invention an improved tuning arrangement is provided forheterodyne receivers in which the substantially complete 1929. SerialNo. 374,742.

suppression of the heterodyne interference is accomplished ,while at thesame time a substantially uniform degree of selectivity and fidelity aremaintained throughout the tuning range. In general the tuning systemcombe maintained at a fixed frequency difference above the resonancefrequency of the syntonous receiving circuits and the coupling circuitsuppression frequency at an equal interval above the heterodyne wave.

' By including the wave trap in the coupling circuit, loosely coupled tothe selective circuits, any detrimental effect that it might otherwisehave upon the tuning of the selective circuit is avoided. Moreover, thetrap is located in a low impedance circuit, thus improving thesuppression obtainable. Also, the impedances constituting the couplingcircuit and the wave trap function to maintain a substantially constantdegree of selectivity and fidelity of reproduction throughout the tuningrange.

The invention will be more readily understood from the followingdescription, in connection with the accompanying drawing in which: s

Fig. 1 discloses a portion of a super-heterodyne radioreceiving setembodying the invention and including a special tuning arrangementconnected between a receiving antenna' and a detector within the radioset.

.discharge devices in a receiving set.

Referring to Fig. 1, the antenna 1 is connected through an antennaloading coil 2,

and a tuned circuit 3 including the winding 4- to ground at 5. Acoupling loop 6, containing a wave trap or suppression circuit 7, iscoupled by means of the winding 8 to the winding 1, associated with thetuned circuit 3. The loop 6 also contains a winding 9 which is coupledto a winding 10 included in a second tuned circuit 11.

Tuned circuit 11 includes a fixed inductance and an adjustable condenser26 in addition to winding 10.

The suppression circuit or wave trap 7 comprises a fixed inductance 27and. an ad justable condenser 28.

The detector 12 is indicated diagrammatiially by a block. The tunedcircuit 11 is connected to the detector 12 by means of the conductors 1Gand 17.

The detector 12 is assumed to be connected to appropriate portions ofthe remainder of the superheterodyne receiving set by means of theconductors and 41, which are shown ending in broken lines.

Oscillations are supplied to detector 12 by a generator 20 the frequencyof which is ad justable by means of a Variable condenser 29.

The condensers 2-1 and 26 are preferably identical in construction andare designed to have a range of capacity values such is commonly used intuned circuits of radio receiving sets.

The inductance of the winding 4 is made small in comparison with thefixed inductance 23 and the combined inductance in tuned circuit 3 ismade to be of such a value that by adjusting the condenser 24 the tunedcircuit 3 may be tuned to any frequency within the range for which thereceiving set is designed.

In order that the antenna circuit may have merely a negligible effectupon the tuning of circuit 3, the antenna loading coil 2 is arranged tobe connected to an intermediate point of inductance 23 bymeans of anadjustable tap 30. In addition, the inductance of the loading coil 2 ismade to be large in comparison with the inductance of the winding 1 andthe portion of inductance 23 included between the tap 30 and ground 5.The inductance of the loading coil is also madesufficiently large toresonate with the antenna capacity at a frequency somewhat below thelowest tuning frequency.

The adjustable condensers 28 and 29 are similar in construction tocondensers 24 and 26 but do not necessarily have the same limits orrates of capacity variation. Each condenser is so designed that when itis used with a fixed tuning inductance the resonate frequency of thecombination will vary in direct proportion to the angular displacementof the rotatable portion of the condenser. The four condensers 2 1, 26,28 and 29 are arranged to be adjusted simultaneously by means of asingle tuning handle or shaft which is indicated symbolically by meansof the bar 31.

The detector 12 is assumed to work into an intermediate amplifyingsystem which is selectively responsive to a single fixed band offrequencies. The tuned circuits 3 and 11 are tuned to the frequency of awave which is to be received. The tuned circuits are coupled by means ofthe loop circuit 6 which has this coupling function in addition to itsfunction as a suppression circuit or wave trap. In the latter capacitythe loop circuit 6 discriminates against an interfering wave which maybe present and to which the usual super-hcterodyne receiving set wouldbe sensitive. This frequency selective discriminating action of loop 6is effected by means of a condition of anti-resonance in the wave trapor suppression circuit 7 whereby the loop 6 is effectivelyopen-circuited so as to substantially neutralize or suppress thecoupling between the tuncd circuits 3 and 11.

To receive the desired incoming wave the oscillation generator 20 mustbe adjusted to generate a frequency which will translate or heterodynethe incoming wave, thus deriving therefrom a wave of the frequency towhich the intermediate amplifier is tuned.

To effect heterodyne reception in the usual manner, the frequencygenerated by the oscil later 20 must be adjusted to differ from thefrequency of the incoming wave by precisely the frequency to which theintermediate amplifier is tuned. In general, there are two possiblesettings of the oscillator which will serve, one above and one below thefrequency of the incoming wave. Conversely, for a given oscillatorsetting, two incoming waves may generally be received. The purpose ofthe wave trap is to suppress the undesired lncommg wave.

It will be evident, that in the system which is described above, thereceiving tuned circuits 3 and 11 are required to be maintained in asyntonous condition, while at the same time the oscillator 20 and thewave trap 7 are required to be adjusted so that the oscillator frequencyis always mid-way between the resonant frequencies of the receivingtuned circuits on the one hand and the wave trap on the other. To effectthis result the condensers 2a, 26, 28 and 29 are so proportioned that agiven angular displacement ofthe rotatable members of all fourcondensers results in equal changes in resonant frequency i therespective systems with which the condensers are associated.

By proper adjustment the combined tuning device comprising the tunedcircuits 3 and 11 together with the loop circuit 6 may be made selectiveto a relatively narrow band of frequencies sufliciently wide to includethe two sidebands usually associated with a radio telephone broadcastingor communication channel.

Due to the use of two coupled circuits, the band selected by the tuningarrangement of circuits 3 and 11.

this invention is wider and more sharply defined than the band selectedby a single tuned circuit. The sensitivity of the system within theselected band is also more nearly uniform, due to the use of thecoupledcircuits. All of these characteristics are con ducive to improvefidelity of reproduction of the detected signals, since more of theessential component frequencies of the sidebands are preserved and allcomponent frequencies are amplified to a substantially uniform degree,while undesired frequencies are dis criminated against.

The width of the selected band of frequens cies is fixed at any desiredvalueby adjustin the amount of coupling between the tune This couplinginvolves the loop 6. It has been found by means of computations andexperiments that the width of the selected band of frequencies,expressed as.

a fraction or percentage of the mean selected frequency, issubstantially equalto the percentage of coupling existing betweencircuits 3 and 11. This rule is one which holds when the degree ofcoupling is relatively small and, therefore, when the selected band isrelatively narrow. with respect to the mean selected frequency.

If, for example, it is desired that the select ed band width should beone per cent of the value of the mean selected frequency in the band,the net coupling between circuits 3 and 11 should be adjusted to besubstantially one per cent.

In respect to the function of the loop 6 as a wave trap, it is desirablethat the frequency discriminated against should be determined solely bythe anti-resonant frequency of the trap circuit 7. For this reason it isadvisable that the coupling coils 8 and 9 should introduce very littleadditional reactance into the loop circuit 6. To this end, the couplingbetween coils 4 and 8 and also that between coils 9 and 10 is made asclose as is practicable.

' Referring to Fig. 2, the terminals 33 and 34 represent the points ofapplication'of a means for applying an incoming signal wave. This wavemay sometimes be accompanied by inconnection are not shown, being wellknown in the art. Tuned circuit 3 contains the variable condenser 24 andthe fixed induc tance 23, the latter being coupled directly to winding 8of the loop circuit 6 instead of through winding 4. Winding 9 of loop 6is coupled directly to the fixed inductance 25 in tuned circuitll. A

' It has been found by experiments, that in some cases in spite of duecare in designin the selective circuits associated with asuperheterodyne receiving set there is a tendency for the selected bandwidth to become somewhat greater when the system is tuned to receive thehigher frequencies in the designed operating range. To improve thiscondition it has been found desirable in such cases to employ a fixedanti-resonant circuit 36 including inductance 37 and condenser 38. Thiscircuit is tuned for a frequency slightly above the highest frequency inthe designe operating range.

The tuned circuit 11 is connected to the detector 12. The oscillationgenerator 20 is not shown in this figure but is assumed to be connectedeither to the plate circuit of the detector 12 or to some other suitableportion of the receiving system. The rotatable parts of the condensers24, 26 and 28 are arranged to be moved in unison by means of the handleor shaft 31 to which may also be joined the condenser for tuning theoscillation generator (not shown).

\Vhat is claimed is:

1. n A selective circuit comprising a pair of syntonous tuned circuitshaving their tuning elements mechanically connected for the maintenanceof syntony' and impedance coupling means for saidtuned circuits, saidcoupling means being adapted to maintain a substantially constant widthband transmission characteristic throughout the tuning range, andincluding a variable element mechanically coupled to the tuning elementsof the syntonous circuits, whereby transmission between said circuits issuppressed at a frequency difi'ering by a substantially constant amountfrom the frequency of tuning of said syntonous circuits.

2. A band selective circuit comprising a pair of syntonous tunedcircuits having their tuning elements mechanically connected, andimpedance coupling means for said tuned circuits comprising a variableelement mechanically coupled to the tuning elements of the syntonouscircuits and adapted to neutralize the coupling of said circuits at afrequency differing by a substantially constant amount from theresonance frequency of said circuits.

3. A receiving circuit comprising a pair of syntonous tunedcircuitsadapted to be tuned to receive an incoming signal, a local heterodyne'wave source, variable means for controlling the frequency of saidsource, elec trical coupling means for said tuned circuits,

a variable impedance included in said cou-, plingmeans whereby thecoupling of said tuned circuits may be neutralized, and means formechanically coupling the tuning elements of said syntonous circuits,the frequency controlling means of said wave source, and the saidvariable coupling impedance whereby the resonance frequency of saidtuned circuits and the frequency at which the coupling of said circuitsis neutralized are maintained at equal intervals above and below thefrequency of said wave source.

l. A receiving circuit in accordance with claim 3 in which the couplingmeans is adapted to maintain an effectively constant width b andtransmission characteristic.

5. In a heterodyne receiving system, a selective circuit comprising apair of syntonous tuned circuits, impedance coupling means for saidcircuits of sufiiciently close coupling effect to promote bandtransmission, and means included in said coupling means for thesubstantial suppression of heterodyne interference.

6. A selective circuit comprising a pair of syntonous tuned circuitshaving their tuning elements mechanically coupled for the maintenance ofsyntony, and impedance coupling means adapted to provide a bandselective transmission characteristic and to suppress transmissionbetween said circuits at a frequency differing by a substantiallyconstant amount from the frequency of tuning of said syntonous circuits.

7. A selective circuit comprising a pair of syntonous circuits havingtheir tuning elements mechanically connected for the maintenance ofsyntony and impedance coupling means for said circuits, said couplingmeans including an anti-resonant circuit adapted to suppresstransmissionbetween said circuits at a frequency differing by asubstantially constant amount from, the frequency of tuning of saidsyntonous circuits.

8. A selective circuit comprising a pair of syntonous circuits havingtheir tuning elements mechanically coupled for the mainte- -nance ofsyntony, and impedance coupling means for said tuning circuits, saidcoupling means being adapted to maintain a substantially constant widthband transmission characteristic throughout the tuning range, andincluding an anti-resonant circuit, said antiresonant circuit having avariable element, mechanically coupled to the tuning elements of thesyntonous circuits, whereby transmission. between said circuits issuppressed at a frequency differing by a substantially con stant amountfrom the frequency of tuning of said syntonous circuits.

9. A selective circuit comprising a pair of syntonous tuned circuitshaving their tuning elements mechanically connect-ed for the maintenanceof syntony, and impedance coupling means for said tuned circuits, saidcoupling means being adapt-ed to provide a band selective transmissioncharacteristic and including a variable element mechanically coupled tothe tuning elements of the syntonous circuits, whereby transmissionbetween said circuits is suppressed at a frequency differing by asubstantially constant amount from the frequency of tuning of saidsyntonous circuits.

10. A selective circuit comprising a pair of syntonous tuned circuitshaving their tuning elements mechanically coupled for the maintenance ofsyntony, and impedance cou pling means for said tuned circuits, saidcoupling means being adapted to provide a band selective transmissioncharacteristic and to suppress transmission between said circuits at afrequency differing by a substantially constant amount from thefrequency of tuning of said syntonous circuits.

11. A selective circuit comprising a pair of syntonous circuits havingtheir tuning elements mechanically connected for the maintenance ofsyntony, and electrical coupling means for said tuned circuits, saidcoupling means being adapted to provide a band se lective transmissioncharacteristic, and. including two anti-resonant circuits for regulatingsaid transmission characteristic, one

f said anti-resonant circuits having a variable element mechanicallycoupled to the tuning elements of the syntonous circuits, wherebytransmission between said circuits suppressed at a frequency differingby a substantially constant amount from the frequency of tuning of saidsyntonous circuits.

In witness whereof, I hereunto subscribe my name this 29th day of June,1929.

HORACE T. BUDENBOM.

